History of Flight AF447 Puzzles Experts

 - June 6, 2011, 6:48 AM
Data found in the flight recorders indicate that the crew maintained pitch-up control inputs whereas the aircraft was already stalled. (Photo: BEA)

French BEA accident investigators on May 27 released factual information they have found in reading data from Air France Flight 447’s recorders, in hope of quenching speculation about responsibilities in the accident. But the information exposed an intriguing sequence of actions in the cockpit.

According to the BEA document, titled “Update on investigation,” the crew flying the Airbus A330-200 continued making pitch-up inputs during the first part of its final descent, during which the aircraft remained stalled. The June 1, 2009, crash in the Atlantic Ocean killed all 228 on board.

One question lies with the trimmable horizontal stabilizer (THS). The critical phase of the flight, from autopilot disengagement to the crash, lasted 4 minutes 23 seconds. During the last 3 minutes 30 seconds, the position of the THS went from 3 degrees to 13 degrees nose-up and then remained unchanged. Yet, from about 2 minutes before the crash, the pilot flying switched to pitch-down inputs.

So, as French website aerobuzz.fr pointed out, why did the THS stay in such a nose-up setting? This may hint at the flight control law being no longer “normal” but in a mode (“alternate” or “abnormal”) where the autotrim function is deactivated. In that instance, the crew has to trim the stabilizer manually. In an A320 accident that took place near Perpignan, France, in 2008, the crew’s failure to recognize this situation contributed to the catastrophic chain of events. In the case of AF447, the crew did mention “alternate law” in the recorded conversation.

The information released May 27 is far from complete–for example, still unknown is exactly what the crew could see on the cockpit displays. The number of questions raised by the BEA’s document speaks volumes about the complexity of what happened off Brazil’s coast. The investigators arguably need months to reach a fair understanding.

BEA started analyzing AF447’s cockpit voice and flight data recorders on May 14 at its Le Bourget laboratories. The media and even France’s minister of transport quickly pressured the agency for answers, leading to inconsistent “leaks” and an incredible suggestion that the accident’s cause should be determined before the Paris Air Show in mid-June. Hence the BEA’s decision to issue the update before an interim report is released toward the end of July.